1. Field of the Invention
This invention relates to the use of a catalytic preburner for heating the air from compressor discharge temperatures to above the light-off or extinction temperatures of a catalytic combustor. Such structures of this type, generally, eliminate the diffusion flame and produce a preburner/catalytic combustor system capable of achieving less than 1 ppm NO.sub.x.
2. Description of the Related Art
The rate of the thermal NO.sub.x production in gas turbine combustors is a function of temperature, pressure, and residence time. For example, under typical gas turbine combustor conditions, thermal NO.sub.x may form in significant concentrations at at high temperatures, (e.g., 1600.degree. C. (2912.degree. F.)). Thus, to prevent thermal NO.sub.x formation, one must avoid operation at high temperatures by premixing the fuel and air so that the adiabatic flame temperature is maintained at lower temperatures. The lean fuel/air ratios needed to satisfy this criteria, however, produce a fuel/air mixture that is difficult to burn given the constraints found in gas turbine combustors (e.g., pressure drop and residence time considerations). In catalytic combustors, this lean fuel/air mixture is burned by using heterogeneous catalytic surface reactions to promote and stabilize homogeneous gas phase reactions.
Since 1970, many studies have been made of catalytic combustors in controlled experiments. Despite a significant research effort, however, catalytic combustors have not yet been applied to gas turbine applications. One of the main obstacles is that under the severe operating conditions found in gas turbine combustors (i.e., relatively low air inlet temperatures, high pressures, high flow rates and low residence time), no catalyst has been developed that is active under the required operation conditions.
Recently, a catalytic reactor has been demonstrated with an extinction temperature of approximately 454.degree. C. (850.degree. F.). These threshold temperatures are, however, above the compressor discharge temperature (e.g. 350.degree. C. (662.degree. F.)). In order to use this catalyst, the preburner must always be used to heat the incoming air from compressor discharge temperatures (350.degree. C.) to the higher reactor light-off and extinction temperatures.
The present preburners, however, are based on a conventional diffusion flame that produces NO.sub.x. This is inherent in diffusion flames as a hot flame front is formed between the fuel and air. The present idea seeks to replace the diffusion flame in the preburner by a catalytic combustor (i.e., a catalytic preburner).
It is apparent from the above that there exists a need in the art for a catalytic preburner which heats the air from the compressor discharge temperatures to above the light-off or extinction temperatures of a catalytic combustor, and which at the same time eliminates the use of a conventional preburner, but at the same time is capable of achieving less than 1 ppm NO.sub.x. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.